The instant invention is directed to a method for laser welding a very thin metal foil to the edge of a metal sheet to achieve a seal, even under vacuum conditions. The method is specifically directed to the welding of very thin metal foils which are less than 0.002 inches thick to such metal sheet. Using the teachings of the instant invention the ultra-thin metal foils are welded to the metal sheet using a laser cutting and welding operation. That is, the foil is welded while it is simultaneously cut and trimmed by a laser such as that of the Neodymium: YAG type. The invention further contemplates using such a method to manufacture a sealed vacuum insulation panel where the use of thin foils is required to minimize thermal conductance.
It is well recognized that the welding of metal foils having a thickness of less than 0.002 inches cannot be achieved using laser welding methods. In fact, the prior art specifically teaches away from using a laser to achieve such welding of very thin metal. For example, U.S. Pat. No. 4,798,931, to Hess, III directed to "Simultaneously Cutting and Welding Sheet Metal Using Laser Energy", specifically states that the laser welding method of the patent will not operate with extremely thin layers, and the patent is directed to metals with a range of thickness from 0.002 to 0.010 inches thick. Other prior patents recognize the difficulties encountered by welding even thicker metals. U.S. Pat. No. 4,546,230 to Sasaki displays graphically that the "welding of metal pieces is difficult unless the thickness thereof is 0.2 mm (0.008 inches) or above, (column 5). Similarly, U.S. Pat. No. 4,905,310 to Ulrich recognizes that a laser cannot be used to weld and cut very thin sheets and foils of metal because "such extremely thin layers may vaporize and/or curl and not function properly", (column 5).
Electrical resistance welding processes have also been used for the welding of thin metal, however, such a method has proved suitable for metals of 0.003 inches or greater. Also, it is believed that laser techniques would prove to be more economic than electrical resistance welding and more able to provide a thicker, stronger, vacuum-tight, or error free weld in metal foils.
In the past, laser technology has been used with plastic and resin films. Such films have been simultaneously cut and welded using laser techniques. Although a laser has been successfully used with such materials, it is not suggested in the prior art that the successful welding and cutting of very thin metal foils can be achieved by using similar methods. U.S. Pat. No. 4,945,203 to Soodak et al is an example of the use of a laser to simultaneously cut and weld plastic film bags. The reference uses a carbon-dioxide type laser to weld Teflon brand films having a thickness on the order of 0.001 inch to 0.10 inch. Even with the knowledge of the Soodak et al reference, it is well known that the success of the method is limited to plastic type of material and would not be successfully extended to very thin metals or foils. Plastic materials have physical and chemical properties which are extremely different from metals. Plastics, for example, can be cut and welded in air at relatively low temperatures, whereas metals must be heated to above their generally very high melting temperatures in order to accomplish a weld.
Similarly, the U.S. Pat. No. 4,500,382 to Foster describes a method for simultaneously cutting resin film to the desired shape while simultaneously welding the edges. Again the prior art recognizes that such a method does not extend to very thin metal foils and teaches away from using such a method with ultra-thin metal material.
Neodymium: YAG lasers and other solid state lasers, such as that used in the process of the instant invention, are well known and have been used for welding and cutting. U.S. Pat. No. 4,461,945 to Ward shows the use of a laser for such a purpose. However, the reference does not disclose the use of the laser for the simultaneous cutting and welding of very thin or ultra-thin metal foils to a metal sheet. U.S. Pat. No. 4,564,736 discloses the use of a Neodymium: YAG laser in cutting, drilling, and welding processes, and U.S. Pat. No. 5,187,967 also discloses the use of a Neodymium: YAG laser for cutting. Again, none of the above cited patents utilize such a solid state laser for the simultaneous welding and cutting of very thin metal foils to a metal sheet. None of the references recognize that such a procedure can be achieved with a pulsed Neodymium: YAG laser.
It is recognized that simultaneous cutting and welding can be achieved utilizing laser technology. As noted above, both U.S. Pat. No. 4,500,382 to Foster, and U.S. Pat. No. 4,945,203 to Soodak disclose such a process with respect to resins or thin plastics although a vacuum fight weld or seal is not achieved. Likewise, simultaneous cutting and welding of metals is disclosed by U.S. Pat. No. 4,798,931 to Hess III, and U.S. Pat. No. 4,905,310 to Ulrich. The Hess III, and Ulrich patents, however teach that such methods do not work with very thin metals or foils.
Specifically, the Hess III, and Ulrich patents do not use a Neodymium: YAG laser, but rather a carbon-dioxide laser as is well known. The two patents disclose a method which "depends upon the creation of a globule or pool of molten metal, some of which is blown away and the remainder of which is used to weld". The patents prefer space between the layers in a controlled manner, and rely on this space for correct operation of the method. A narrow jet of air or inert gas is also required by the references to blow away some of the molten metal although some is left at the laser site. The spacing between the two metals to be welded is such that a globule of molten metal will bridge the gap and form a weld. In these two patents the focal point of the laser is set to be at the surface of the work, or below the surface. The patents suggest that for relatively thin materials, the focal point be set at the surface of the work. The preferred embodiment of the patents is concerned with the welding of 321 stainless steel with a thickness of either 0.006 inches or 0.004 inches. No disclosure is made of an effective method for welding foils with a thickness of less than 0.002 inches.
As will be seen from the following description, the process of the instant invention varies from that of the Hess HI and Ulrich patents to satisfy a long felt need and to achieve an improved and unexpected result.
The instant invention is directed to the use of an ultra-thin metal foil to seal between two metal plates while forming a vacuum insulation panel. Prior art insulation is shown in U.S. Pat. No. 5,157,893 to Benson et al. The panel comprises two adjacent metal sheets spaced close together with beads or spacers there between. Gases are evacuated from the panel to form a vacuum In the prior art device the edges of the metal plates were sealed together. It has been found that the instant invention provides a method for manufacturing such panels with ultra-thin foil as an edging between the plates. The thin metal foil edge seals greatly improve the thermal insulating performance of the panels by reducing the thermal conductance between the hot side and the cold side of the insulation.
Welding has also been used for sealing purposes as shown in U.S. Pat. No. 4,117,300 to Ricards. The patent is directed to a method of hermetically sealing battery containers. The method of Ricards uses both a resistance weld and a laser weld. The patent, however, is not directed to the use of ultra-thin foils and the problems associated therewith.